山东邹平王家庄铜(钼)矿床蚀变围岩中含云母流体包裹体的成因及其意义

Origin and significance of mica-bearing fluid inclusions in the altered wallrocks of the Wangjiazhuang copper-molybdenum deposit,Zouping County,Shandong Province

王家庄铜(钼)矿床位于鲁西邹平火山岩盆地。矿床围岩,即王家庄复合岩体,主要由闪长岩、二长岩和石英二长(闪长)岩等组成。从岩体周边向内到矿体,依次出现钾蚀变带、钾硅化蚀变带和强钾硅化蚀变带。矿床主要有两种类型矿石:深部蚀变石英二长(闪长)岩中的早期浸染状矿石和浅部角砾岩化石英二长(闪长)岩中的块状硫化物石英脉矿石。矿石矿物主要有黄铁矿、黄铜矿、砷黝铜矿和辉钼矿等。流体包裹体研究表明,除了证实有前人确定的富液相包裹体、富气相包裹体和含石盐的高盐度包裹体外,还在钾硅化蚀变-矿化围岩中发现含云母的流体包裹体。它们多随机或成群分布于石英晶核和生长环带中。岩相学、拉曼光谱和电子探针分析表明,含云母的流体包裹体主要由水液相+气相+白云母(黑云母)组成,其相比例随石英晶体不同或是相对稳定,或是有很大变化。显微测温表明,含云母的流体包裹体中水溶液的冰点和均一温度有很大变化,而主要集中在-5～10℃和310～360℃。热液金刚石压腔(HDAC)加热实验显示,含云母流体包裹体的均一温度可高达773～790℃,由此估算的均一压力高达275～380MPa,将大大超过地质推断的合理范围。对含云母流体包裹体的成因机制进行了讨论。认为它们不属于熔融包裹体范畴,而是钾硅化蚀变阶段捕获的原生流体包裹体,是从中酸性岩浆结晶作用晚期分离出来的富K-Al-Si组分、挥发分、盐类和成矿金属元素的含水流体中非均匀捕获的产物。伴随裂隙开放和压力温度迅速降低,导致大量白云母(少量黑云母和硅酸盐等)从富K-Al-Si组分的含水流体中结晶出来,沉淀在正在结晶的石英核部或生长表面坑穴,随后与石英周围的流体介质一起被捕获,成为含云母流体包裹体。含云母流体包裹体属于岩浆作用晚期自交代作用产物,它们与含石盐包裹体的共存反映了这两种矿物在岩浆-热液中达到饱和并沉淀出来,对于流体流动和成矿作用具有重要意义,并有助于我们对王家庄铜(钼)矿床成矿流体的性质和岩浆-热液演化过程有深入了解。

The Wangjiazhuang copper-molybdenum deposit is located in the Zouping volcanic basin in western Shandong. The wallrock of the deposit,the Wangjiazhuang composite intrusion consists mainly of diorite,monzonite and quartz monzonite. From the periphery of the intrusion towards inner zone occur potassic,potassic-silicic and intensive potassic-silicic alteration zone in tern. The deposit contains mainly two types of ores: stockwork-/disseminated Cu-Mo sulfides in the altered quartz monzonite and coarse and wellcrystallized sulphide-quartz veins in the brecciated quartz monzonite. The ore minerals include mainly pyrite,chalcopyrite,tennantite and molybdenite. Fluid inclusion study has shown that the deposit contains not only liquid-rich,vapor-rich aqueous and halite-bearing inclusions which were recognized in the quartz vein ores by previous researches,but also mica-bearing fluid inclusions in mineralized wallrocks with potassic-silicic alterations. These inclusions either occur randomly in quartz,or are densely distributed in the cores or growth zones of quartz crystals. Petrography,Raman spectroscopic and microprobe analyses indicate that the mica-bearing fluid inclusions contain mainly aqueous liquid,vapor and muscovite( or biotite); the phase ratios in the mica-bearing fluid inclusions vary considerably in quartz crystals. However,in some individual quartz crystal the phase ratios of these inclusions are relatively stable.Microthermometry has shown that both the ice-melting temperatures( Tmi) and homogenization temperatures( Thaq) of aqueous phases in the mica-bearing fluid inclusions vary considerably but with a mode in the range of-5 ~ 10℃ and 310 ~ 360℃,respectively.Preliminary heating experiments of mica-bearing fluid inclusions in hydrothermal diamond anvil cell( HDAC) indicate that the total homogenization temperatures are as high as 773 ~ 790℃,and the pressure thus estimated could be 275 ~ 380 MPa that would be too high to the reasonable range from geological consideration. The origin and formation mechanism of mica-bearing fluid inclusions are discussed. It is suggested that the mica-bearing fluid inclusions belong to primary fluid inclusions rather than melt inclusions. They were heterogeneously trapped from an aqueous fluid rich in K-Al-Si components,volatiles,salts and metals that was separated from a crystalline intermediate-acidic magma. With the opening of fractures and quick drop of pressure and temperature of the rock system,muscovite( biotite and some other silicate minerals) would crystallize from aqueous fluid rich in K-Al silicates and deposit in the core or growth surface of crystallizing quartz grains and then were trapped with surrounding fluid media as mica-bearing fluid inclusions.Mica-bearing fluid inclusions belong to the products of authi-metasomatism in the late magmatic processes. The coexistence of micabearing and halite-bearing fluid inclusions implies that both minerals were saturated in the magmatic-hydrothermal solutions and deposited that has important significance in terms of fluid flow and mineralization.